Behaviour of rubberized cement-bound aggregate mixtures containing 1 different stabilization levels under static and cyclic flexural loading

11 An investigation has been undertaken to investigate the influence of rubber inclusion at 12 different levels of stabilization on the behaviour of cemented granular mixtures, under static 13 and cyclic flexural testing, and to compare this with mixtures without rubber. Both are intended 14 to be used as base courses of semi-rigid pavement structure. 3%, 5%, and 7% of cement by dry 15 weight of aggregate were used for stabilization purposes. Rubberization of cemented aggregate 16 was conducted by replacing 30% of the aggregate of the 6 mm fraction size by an equivalent 17 rubber volume. The investigated properties were flexural strength, static and dynamic stiffness 18 moduli, toughness and fatigue life. Damage due to cyclic loading was evaluated in terms of 19 stiffness degradation and permanent deformation accumulation. Flexural-induced cracking 20 behaviour was also investigated. Results reveal that the rate of flexural strength increase is 21 higher for the reference cemented mixtures. As stabilizer quantity increase, both static and 22 dynamic stiffness moduli increased while rubberization mitigated these two parameters at all 23 stabilizer contents. Toughness and fatigue life were improved due to rubber modification at 24 investigated stabilizer contents. Flexural-induced cracks always tend to propagate through 25 rubber aggregate regardless of the quantity of cement.

nature of rubber aggregate particles. Therefore, some attempts were conducted to achieve such 47 adhesion improvement. These are using sodium hydroxide to ensure proper roughness of rubber 48 and also improving the interaction between rubber and other components by using silica fume. 49 The findings indicated some improvement in performance. As pavement structures consumes large quantity of natural materials, replacing the latter 52 materials with rubber aggregate will ensure a higher degree of sustainability compared with 53 other applications (Cao 2007, Barišić et al. 2014). This may be the reason that many studies  One of the most important steps necessary to allow the beneficial use of waste tires within 70 cement-bound pavement layers is to understand how the inclusion of these waste materials at 71 different cement contents may influence the final behaviour when subjected to flexural loading.

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Flexural testing was selected due to its close simulation of field conditions. As pavement 73 structures are subjected to cyclic loading due to the movement of vehicles, the behaviour was 74 also evaluated under cyclic loading. Cracking in cement-bound mixtures usually develops due to tensile stresses. As cracking 77 represents the main disadvantage of using cemented layers due to reflection of these cracks 78 through the overlying layers, studying the role that cement content plays on cracking tendency 79 and pattern is the key to optimizing mix design. Limited information has been found regarding 80 quantitative investigation of cracking patterns of cement-bound mixtures.   Table   93 1 shows the properties of cement used in this study while Table 2

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These proportions were as follows:11% of 20mm, 20% of 14mm, 11% of 10mm, 13% of 6 mm 103 and 45% of particles less than 6mm. The design aggregate mixture gradation is shown in Figure   104 1. The stabilization of mixtures was conducted using cement content of 3%, 5% and 7% by dry 105 weight of aggregate. These were selected based on the range studied by (Thompson 2001). Due 106 to its size similarity with that of rubber particles, the 6 mm size was selected for partial 107 replacement by rubber to ensure the same aggregate packing. Therefore, rubberized mixtures 108 were constituted by replacing 30% of the volume of this fraction by an equivalent volume of 109 tire-derived rubber aggregate which equals about 3.9% by the volume of the total aggregate 110 mixture. The design water quantity for each cement content were adopted from a previous study  . In this procedure, cement was added first to the fine dry aggregate material, 121 then the product was mixed for 60 sec. with other aggregate fraction sizes including rubber 122 aggregate. Then, a mixing for 120 sec. was conducted after adding the design moisture content.

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Oiled prismatic steel moulds of 100mm x 100mm x 500mm dimensions were used for 124 specimens' fabrication. Once the mixing was finished, a vibrating hammer was utilized to 125 compact the mixture inside the prepared moulds in three layers. Each compacted layer was 126 scarified before compaction the next layer. To ensure accurate characterization, previous tested and the average value was presented. Once compaction was achieved, specimens were 131 left covered in their moulds to prevent moisture loss. Then, these were demoulded on the next 132 day, wrapped with cling film and stored in wet plastic bags for the curing period (28-days).  Regarding instrumentation and data acquisition, an external high-speed data acquisition device 173 was used since the idea was to identify the behaviour under cyclic flexural loading in terms of 174 damage accumulation during fatigue testing. To this end, the load-mid span dynamic deflection 175 data was acquired at a rate of 20 points per second since this would ensure sufficient data to 176 describe a load-deflection cycle as reported by Sobhan (1997). Figure 3 shows the cyclic 177 loading test setup.

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In this paper, the fatigue life was defined by the full collapse point since, in the case of stress-180 controlled fatigue testing, there is negligible difference between a stiffness reduction criterion 181 and the full collapse point (Li 2013 Table 3 shows densities of prisms manufactured from different amounts of rubber and cement.

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As clearly seen from Table 3, the cement content has a slightly positive impact on the density 209 of compacted mixtures due to its filling of the voids between larger particles. increased from 3% to 5% while little improvement occurs beyond that amount of stabilization.

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The rate of microcracking development in concrete specimens under cyclic load was observed 318 non-destructively by Shah and Chandra (1970). Their results indicated that the microcracking 319 development was stable and slow in the case of 70% and below stress ratio whereas at 80% and 320 higher, the rate of microcrack growth was faster. Such a conclusion is consistent with the 321 current findings.

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Permanent deformation is also a stress ratio-dependent parameter as illustrated in Figure 12.   As can be seen from Figure 15, for all cement contents, the amount of rubber observed on the 382 fractured surface was greater than that originally used in the preparation of the rubberized 383 sample. This indicates that the propagation of cracks preferentially occurred through the rubber 384 aggregate. However, at low cement content (3%), the difference between the two rubber 385 quantities was small as compared with that at 5% and 7%. This indicates a reduced tendency 386 for cracks to propagate through the rubber particles. It seems that the reason behind this 387 phenomenon is that, at 5% and 7%, the rubber particles were much the weakest points in the